Ohio Rust in closed loop

I have a FHP that is about 10 years old. When I installed it the local HVAC people that were recommended by FHP did the loop, then came back and flushed and filled it. I have a Flow Center ground loop pump set that has two Bell & Gossett PL-36 cast iron circulating pumps in it. One pump developed a leak in the cast iron housing so I pulled it apart. The water that spilled out is full of rust and the brass fittings have some rust built up on the inside of them. The liquid in the system has no smell or color. I am wondering if the guy that filled it put in only water? I am also wondering if the rust will corrode the heat exchanger? Should I flush it out? If so what chemical should I use for antifreeze? The Flow Center has brass ports that I can use to pump water through it and then fill it with antifreeze.

You should not have rust unless there is metal contact with water. I would find the source of the metal and get rid of it. Brass or copper not counted of course. I use methanol or washer fluid that has methanol in it.

Thanks for the reply. The pumps in the Flow Center are cast iron and are the source of the rust. I replaced the pump that had developed the pinhole leak. I am thinking that if I don't use something like propylene glycol they will continue to rust. There is a brass version of this pump that is intended for potable water. What type of washer fluid do you use? Is it designed for cars?
Thanks: